1. Field of the Invention
The present invention relates to a liquid crystal device driving circuit, and more specifically, to a circuit for driving a liquid crystal display panel capable of displaying an image with a multiple tone level.
2. Description of Related Art
In general, since the liquid crystal display panel is configured to be able to display an image having different gray scale levels, a circuit for driving the liquid crystal display panel is required to generate a plurality of different driving voltages of the number corresponding to the number of gray scale levels to be displayed. Therefore, the driving circuit is configured to receive image data to be displayed, and to perform various shifting and latching processings for the received data, and then to sequentially select one driving voltage level from the plurality of driving voltage levels in accordance with a latch output, so as to supply the selected driving voltage level to the liquid crystal display panel.
Here, referring to FIG. 1, there is shown a block diagram of one example of a conventional liquid crystal device driving circuit. As shown in FIG. 1, in the case of displaying a gray scale image in an active matrix liquid crystal display panel, the conventional liquid crystal device driving circuit has to supply a drive output voltage Vo corresponding to a required luminance, from a group of drive voltage output terminals Tl to Tk of a transistor switch circuit 3 to corresponding source lines.
For this purpose, a select/drive circuit includes "k" stages of "n"-bit shift registers 15a to 15k receiving an image input data from an image data input terminal, and a corresponding number of "n"-bit latches 16a to 16k each for latching the "n"-bit data of a corresponding one of the "n"-bit shift registers 15a to 15k. A group of selector circuits 14a to 14k are driven with select signals from the latches 16a to 16k. If the group of selector circuits 14a to 14k are driven, a gate of each of output transistors Qll to Qmk included in the transistor switch circuit 3 is controlled, so that a driving output voltage Vo is supplied from each of the driving output terminals Tl to Tk.
Namely, an "n"-bit digital image input data Vi indicative of "m" gray scale levels is supplied from the image data input terminal 7, and shifted and stored in the "n"-bit shift registers 15a to 15k in response to a clock pulse CK applied to a clock input terminal 1. In response to a latch pulse L applied to a latch pulse input terminal 2, the data stored in each of the registers is transferred to a corresponding one of the "n"-bit latches 16a to 16k.
The data latched in each latch controls a corresponding one of the selector circuits 14a to 14k in such a manner that one transistor of the first output stage transistors Qll to Qml connected to the drive output terminal Tl of the transistor switch circuit 3 is turned on, and one transistor of the "k"th output stage transistors Qlk to Qmk connected to the drive output terminal Tk is turned on. With this arrangement, voltage levels Vl to Vm corresponding to drain voltage terminals 8a to 8m of "m" gray scale levels are supplied, so that voltages of "m" gray scale levels are supplied to an external liquid crystal display.
For example, assuming that tit image input data Vi is composed of digital signals D0, D1, . . . , Dn-1, the voltage Vo appearing on the drive output terminal Tl is as shown in FIG. 2.
Namely, if the digital signals D0, D1, . . . , Dn-1 of the image input data Vi are (0, 0, . . . , 0), the transistor Qll connected to the output terminal Tl is turned on, and then, the output voltage Vo is brought to V.sub.l. Accordingly, any one of the output transistors Qll to Qml corresponding to the digital signals D0, D1, . . . , Dn-1 of the image input data Vi is necessarily turned on.
In this approach, if it is attempted to increase the number of gray scale levels, the number of gray scale voltage supplies must be correspondingly increased, and the number of switches must also be increased. Therefore, not only a peripheral circuit becomes large, but also a chip size becomes large in the case that the driving circuit is implemented on a LSI (large-scaled integrated circuit). Accordingly, realizability drops.
Because of this reason, a liquid crystal device driver adopting this approach, which is now under a mass production, is on the order of 8 gray scale levels to 16 gray scale levels. For a full-color display, however, the liquid crystal display panel is going to be required to have a gray scale of 64 levels or more. Under this circumstance, in order to increase the number of gray scale levels, for example, Japanese Patent Application Laid-open No. Sho 63-182695 has proposed a field division method.
In this method, for example, the image is divided into a first field and a second field, and different voltages or the same voltage is generated so that the number of gray scale levels is apparently increased. In this conventional example, in a liquid crystal device driving circuit configured to select one from gray scale voltages in accordance with the image data by means of switching means similarly to the first mentioned conventional example, a "m"-bit image signal obtained by giving a difference of "1" to the most significant "m" bits of a "q"-bit image data ("q"&gt;"m") is generated by a compensation circuit. In addition, each frame of the input image signals is divided into "2.sup.(q-m) " fields, and either the most significant "m" bits of the input image data or the "m" bits outputted from the compensation circuit is selected by a selecting means in accordance with the least significant "q-m" bits of the image data, and the selected "m" bits arc supplied to the liquid crystal device driving circuit.
The compensating circuit either adds "1" to the value of the "m" bits or subtracts "1" from the value of the "m" bits. In the case of addition of "1", only the output of the compensating circuit corresponding to the least significant "q-m" bits of 2.sup.(q-m) fields of one frame is supplied to the driving circuit, and on the other hand, the most significant "m" bits of the input image data is supplied directly to the driving circuit for the remaining portion.
In the case of subtracting "1" from the value of the "m" bits by the compensating circuit, only the output of the compensating circuit corresponding to the least significant "q-m" bits of 2.sup.(q-m) fields of one frame is supplied to the driving circuit, and on the other hand, the most significant "m" bits of the input image data is supplied directly to the driving circuit for the remaining portion. With this feature, the number of gray scale levels can be apparently made to 2.sup.q. For example, in the case of q=4 and m=3, namely, when it is intended to obtain 16 gray scale levels with switches each having eight outputs, there is obtained a relation as shown in FIG. 3.
In the conventional liquid crystal device driving circuit shown in FIG. 1, if the number of gray scale levels is increased, low-impedance large-current-capacity, external voltage supplies of the number corresponding to the number of gray scale levels are required, and therefore, when the driving circuit is assembled in the liquid crystal display panel, the number of thick wiring conductors is increased and the liquid crystal display panel correspondingly becomes large. In addition, by increasing the number of pixels in the liquid crystal display panel, the driving circuit is required to have a further low impedance. Furthermore, if the number of gray scale levels is increased, when a buffer circuit having a low impedance and a large output capacity is implemented on the same semiconductor substrate, the chip size becomes extremely large, and therefore, the driving circuit becomes expensive.
On the other hand, the liquid crystal device driving circuit disclosed in Japanese Patent Application hid-open No. Sho 63-182695 makes it possible to obtain a multiple gray scale with a reduced number of driving circuits by the field division. But, the image quality drops. Namely, when an intermediate gray scale or half tone is generated with the compensating bits, the frame period of the half tone lowers. In addition, a flicker phenomenon and/or a moving phenomenon occurs. Because of these phenomena, the image quality drops.
Therefore, in order to drive the liquid crystal device with an excellent image quality, the liquid crystal device driving circuit itself has a function of supplying different-output voltages of the same number as that of gray scale levels during one horizontal period. If it is so modified, the problems of the conventional liquid crystal device driving circuit shown in FIG. 1 will occur.